I'm surprised that async/await is the preferred idiom.
Having worked with async/await in Node.js a lot, it is of course a significantly better solution than plain promises, but it is also quite invasive; in my experience, most async code is invoked with "await". It's rare to actually need to handle it as a promise; the two main use cases where you want to handle the promise as a promise is either when doing something like a parallel map, or when you need to deal with old callback-style code where an explicit promise needs to be created because the resolve/reject functions must be invoked as a result of an event or callback.
Would it not be better to invert this -- which is the route Erlang and Go went -- and make it explicit when you're spawning something async where you don't want to deal with the result right away? In Go, you just use the "go" keyword to make something async. So the caller decides what's async, not the callee. If callers arbitrarily decide whether to be async or not, a single async call ends up infecting the whole call chain (which need to be marked "async" unless you're explicitly handling the promise/continuation without "await").
Both Erlang's and Go's concurrency model is thread-based. An "async" operation invokes a function on a separate call stack. The function doesn't need to know which call stack it's invoked on. The functions that that function calls don't need to know, either. Lua's coroutines are similar.
The problem with the above approach, however, is that you can't create hundreds of thousands of threads while also transparently supporting the traditional C ABI. C ABIs aren't designed to dynamically grow the stack, and so any thread that needs to invoke C (or Objective-C) code must always create threads with very large stacks (on the order of hundreds of several hundred KB or even megabytes) if they want to support legacy code.[1]
Languages that don't want to put the effort into growable stacks have no choice but to implement a solution that requires annotating the function definition, directly or indirectly.[2] The annotation tells the compiler to generate code that stores invocation state (temporaries, etc) on a dynamically allocated call frame (usually allocated by the caller) rather than pushing them onto the shared thread stack. This is true whether or not the function is a coroutine that can yield multiple values before finishing. Basically, without using a thread-based model, you can never put the caller in full control.
[1] Work on GCC Go necessitated adding a feature to GCC called split stacks. So GCC can actually compile C and (I think) C++ code that can dynamically extend their stacks. However, for it to work properly you have to compile everything with split stacks, including libc and all dependent libraries.
[2] Technically a compiler could emit two versions of a function, one that uses the thread stack for temporaries, and one that uses a dynamically allocated frame. That would put the caller in control. Some languages with very complex meta-programming capabilities (like various Lisps) can do this by making the await keyword a function which literally re-writes the callee into an async function that stores temporaries on a caller-provided buffer. So you can implement it without any compiler support. But it's still limited because the functions invoked by the async function would have to be rewritten recursively. The thread-based design is really the best approach, but it's a non-starter for many languages because of concerns about interoperability and legacy support. JavaScript rejected a thread-based model because existing implementations were too heavily dependent on the semantics of the traditional C stack, and they didn't want to throw away their existing investments.
> The problem with the above approach, however, is that you can't create hundreds of thousands of threads while also transparently supporting the traditional C ABI. C ABIs aren't designed to dynamically grow the stack, and so any thread that needs to invoke C (or Objective-C) code must always create threads with very large stacks (on the order of hundreds of several hundred KB or even megabytes) if they want to support legacy code.
Where did this claim originate from? It gets tossed around all the time in greenthread discussions but it's completely false. When you create a real thread even though it has an 8MB stack or whatever it doesn't actually allocate 8MB. 8MB is not the allocation size, it's the growth limit. The stack grows dynamically allocating memory when necessary until it hits that limit. The C/C++/<insert any language here> ABI doesn't need to be compiled for this because it's just page faulting. Standard OS behavior for decades.
To your second point, your lisp analogy requires a macro or very sophisticated JIT. These are not viable in swift's primary runtime, not even a little bit. It's a non-starter.
> Node.js [ ... ] most async code is invoked with "await"
I think this might be caused by a lack of complementary programming concepts, such as actors.
Having worked with C# and the Orleans actor framework I found that I'm now using more complex async constructs, such as await Task.WhenAll() and async Linq statements that actually make code run more efficient.
Adding async and actors together to the Swift language seems like a very good idea to me.
Async / await also need a idiom to accompany on how to support cancellation. In practice, cancellation happens a lot because the unbounded latency for async operations. Without a throughout support, async / await syntax is probably usable on server side somewhat but still hardly applicable on client side (as the latency is unbounded). On the other hand, C# does go through the pain and added cancellation support to all its standard libraries async API.
I can't upvote this enough. In practice, supporting cancellation is one of the most important and tedious parts of asynchronous programming.
It's easy to start a some task with a completion handler. Even error handling isn't that hard, since communication of a failure goes in the same direction as communicating success.
But cancellation goes into the other direction! So whenever you start an async operation, you need to somehow store a handle or something, so you can cancel it later on.
The actor model makes this even worse -- how do you cancel a command that you previously sent? How do you tell an actor that they don't need to perform an operation, if the operation is still on the queue, or that they should abort the operation if they are already working on it?
If the actor model doesn't have an answer to this problem, developers won't be able to use it as is, and they will have to build additional abstractions on top of it before they can use it.
You create an actor that performs an operation and simply kill it if you want to cancel that operation. Promises/futures with cancellable contexts are equivalent to actors.
I don't think we should get too focused on cancellation.
It's inherently a half-measure: it's used to avoid wasting resources, but it only comes into play after you've already wasted resources. If you want to minimize waste, you're going to do better if you can minimize initiating operations that end up needing to be canceled.
Not that it isn't a useful refinement. It should be planned for. But I don't think it can be considered a critical feature out of the gate.
You are misunderstanding why cancelling is necessary. Cancelling is not for undoing stuff. Cancelling is necessary to avoid waiting for asynchronous operations that have become unnecessary.
Eg. imagine that a document is auto-saved in the background, but the network is busy. Then the user changes the document. Now the app wants to auto-save the new version, but the previous version hasn't been saved yet. Saving the previous version no longer makes any sense and would just waste time, so you want to cancel the previous save operation, and save the new version instead. (it doesn't matter if cancelling the previous save is successful or not -- all that matters is that you don't want to wait for the previous action)
Or imagine that an app opens and tries to show the last document that the user displayed. The document is big, so the app shows a loading indicator. But the user doesn't want to actually view the document, he wants to view a different document, so he closes the document and opens a different one. Now the app needs to cancel loading the first document, because otherwise it would take much longer to load the second document that the user actually wants to see.
As someone who doesn't develop for the Apple ecosystem, I can't quite find one place that articulates well what the Swift development philosophy is and what it brings to the table besides just being modern language with shims for interacting with legacy Apple APIs. Why would I use Swift on Linux?
Most of the posts that I see related to Swift are RFCs evaluating solutions to problems in other languages. I rarely get to see the actual solutions being integrated into the language. Is this just a result of HN readers caring more about language design than learning to leverage the Swift language?
>I can't quite find one place that articulates well what the Swift development philosophy is and what it brings to the table besides just being modern language with shims for interacting with legacy Apple APIs.
It's a modern, fast, statically compiled, statically typed, language (that also has shims for interacting with legacy Obj-C code, but that's not a very important aspect) that unlike Go keeps up with modern PL features and expressibility, and unlike Rust has automatic RC-style memory management that you don't have to think about as much. There's no over-arching principle at play -- it's a pragmatic language.
>Why would I use Swift on Linux?
Because you like the actual language and its ecosystem (or not). It's not like you should be using any language just because of some "design philosophy" it's supposed to have.
>Most of the posts that I see related to Swift are RFCs evaluating solutions to problems in other languages. I rarely get to see the actual solutions being integrated into the language.
Well, there are several free books from Apple and tons of material to see what the language itself offers.
It also has a really slow compiler and that generates code which is about 2x slower than modern C++, which incidentally also has everything you mention (mostly in the form of libraries to prevent bloating the language).
I just wrote a swift api that I'm running on Ubuntu 16.04. I wasn't hesitant to use swift because first, it substantially outperforms node on many benchmarks [1] [2] and second, the community of people who are excited about swift seem to have made up for its young age. You can find workable tutorials and help online for server side Swift. And there are multiple backend frameworks to choose from. Having type safety on the server is as great as it is on the client. It's also great to be able to write the server in the same language as the client. Makes for a VERY smooth dev process. Why wouldn't you use swift on the server? (my guess is fear of new tech -- which can be healthy many times but, IMHO, at least in the case of server side swift, it may be a bit irrational!)
1. Specifically for the first set of benchmarks linked to, they are really irrelevant because they don't represent typical workloads unless you're writing a MAAS (Mandlebrot As A Service). Node was designed for I/O efficiency, not fastest CPU-bound computations. That's exercising the JS engine (e.g. V8) more than anything node-specific. With node becoming more VM-neutral, it's entirely possible for other engines (e.g Chakracore or SpiderMonkey) to be better at other types of computation.
2. Especially in the second set of linked benchmarks, it seems the author of the article was hardly a node.js developer because they not only used an older node branch at the time they wrote the article, but they left out a lot of common optimizations (some of which were pointed out in the comments section). Even Express (which the author used) is known to not be very well optimized.
With that in mind, benchmarks are not the only thing you should be looking at IMHO. For example (for me personally), using a single language for frontend and backend is a big deal because there is less cognitive overhead when switching between the two (previously I often found myself writing JS syntax in PHP scripts and vice versa and trying to remember the APIs for different languages/platforms is difficult). There are many other benefits as well, just watch some of Mikeal Rogers' talks to get a better idea.
As someone who is starting to dabble in the Apple ecosystem via Swift - I heartedly agree. There are plenty of resources for new developers who are trying to learn programming via Swift, but if you are a seasoned developer who wants to jump straight into best practices and architectures while learning the syntax and features at the same time... you're going to have a bad time.
I also feel like with Swift people are so focused on pumping out their mobile app that best practices go out the window, or even that the knowledge is seen as such a valuable and proprietary skillset that the senior and experienced developers simply keep it to themselves and don't publish (compared to other OSS ecosystems)...
Feels like the elephant in the room for the description is not once mentioning Futures but instead jumping straight to using async / await keywords and the actor model.
As evidenced by C#, you can't avoid leaking the type signature of async operations if you actually support generic programming- so while that's a nice ergonomics improvement, it only adds complexity to the actual concurrency model. Go enthusiasts out there will appreciate that go solves this by refusing to support PROGRAMMABLE generic abstractions at all (looking at you, channels and map).
Referencing the actor model and making it first class is interesting, but probably a mistake. Actors are hard to reason about because they're so flexible. Pony is a good recent attempt at combining static types with actors, bit they didn't put performance into the "non-goals" section of their language spec.
If you want task level concurrency and you want it to play nice with your type system, you have to start with Scala and work backward to the alternative implementation choices you're going to make because it checks all the boxes of all the "goals" and ALSO has a very mature actor model implementation that doesn't require promoting actors to keyword status in the language.
Forgive my naivety, but parent article seems to contradict the github post linked in your reply in quite a few ways. What is the time / design iteration relationship between the two, and why (not to put you on the spot)? These are VERY different proposals.
The part where the execution graph is "linked" at runtime into an actor graph. It's hard to reason about because it's flexible enough to enable large numbers of possible combinations.
I'm not saying it's bad, but I am saying that the flexibility limits the ability to reason about actor interactions (especially concurrent interactions), which makes it akin to the dynamic vs static type system debates. One is strictly more flexible than the other, and that necessarily makes it harder to reason about.
Actor systems can be racy and so to reason about them you may need to consider things like the order messages are delivered - which you can't always do.
Maybe it's time for Swift to decide whether it's going to stick to multicore systems or actually leave this single box mindset and enable distributed systems programming. I think designing for distributed systems first could lead to a lot of right choices from the beginning and would still allow for various optimizations later to get the best performance from multicore. Alternative is not very promising and provides a lot of room for really stupid mistakes. Either way, I'm glad to see actor model getting more traction.
Well, multicore isn't going away; concurrency (if not parallelism) is necessary for responsive UIs.
Did you have any specific improvements? The big requirements for distributed programming are mostly protobuf serialization and tcp/http; swift has both already.
I was thinking more in terms of first class support for distributed actors, with global addressing, message routing, etc. Not designing for multicore performance first, the way Pony did.
Definitely some cool ideas here! The motivation seems a little high-level to me, though. Language features ought to help you solve concrete, real-world problems. I can understand what problem async/await solves -- there's a great example with code -- but the actor stuff isn't as clear-cut.
Your comment is rude and factually very wrong. The new languages we discuss are definitely not written by "current hipsters", whatever you meant with that.
How does the OS implement concurrency, which is a userspace problem?
Software interrupts alone are not enough, you still need some language support to it. I am not sure you really know what concurrency even means...
This is another gross misconception, like an attention to manage resources from the userspace, the way JVMs or Node are trying to do by poorly reimplementing OS subsystems.
Concurrency primitives must be implemented by an OS - it will eliminate all the userspace problems - no busy-waiting, no polling, no waking up for every descriptor, etc.
Ignoring an underlying OS instead of having thin wrappers, like Plan9 from userspace does, is exactly what is called ignorance (lack of awareness of a better way).
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Having worked with async/await in Node.js a lot, it is of course a significantly better solution than plain promises, but it is also quite invasive; in my experience, most async code is invoked with "await". It's rare to actually need to handle it as a promise; the two main use cases where you want to handle the promise as a promise is either when doing something like a parallel map, or when you need to deal with old callback-style code where an explicit promise needs to be created because the resolve/reject functions must be invoked as a result of an event or callback.
Would it not be better to invert this -- which is the route Erlang and Go went -- and make it explicit when you're spawning something async where you don't want to deal with the result right away? In Go, you just use the "go" keyword to make something async. So the caller decides what's async, not the callee. If callers arbitrarily decide whether to be async or not, a single async call ends up infecting the whole call chain (which need to be marked "async" unless you're explicitly handling the promise/continuation without "await").
The problem with the above approach, however, is that you can't create hundreds of thousands of threads while also transparently supporting the traditional C ABI. C ABIs aren't designed to dynamically grow the stack, and so any thread that needs to invoke C (or Objective-C) code must always create threads with very large stacks (on the order of hundreds of several hundred KB or even megabytes) if they want to support legacy code.[1]
Languages that don't want to put the effort into growable stacks have no choice but to implement a solution that requires annotating the function definition, directly or indirectly.[2] The annotation tells the compiler to generate code that stores invocation state (temporaries, etc) on a dynamically allocated call frame (usually allocated by the caller) rather than pushing them onto the shared thread stack. This is true whether or not the function is a coroutine that can yield multiple values before finishing. Basically, without using a thread-based model, you can never put the caller in full control.
[1] Work on GCC Go necessitated adding a feature to GCC called split stacks. So GCC can actually compile C and (I think) C++ code that can dynamically extend their stacks. However, for it to work properly you have to compile everything with split stacks, including libc and all dependent libraries.
[2] Technically a compiler could emit two versions of a function, one that uses the thread stack for temporaries, and one that uses a dynamically allocated frame. That would put the caller in control. Some languages with very complex meta-programming capabilities (like various Lisps) can do this by making the await keyword a function which literally re-writes the callee into an async function that stores temporaries on a caller-provided buffer. So you can implement it without any compiler support. But it's still limited because the functions invoked by the async function would have to be rewritten recursively. The thread-based design is really the best approach, but it's a non-starter for many languages because of concerns about interoperability and legacy support. JavaScript rejected a thread-based model because existing implementations were too heavily dependent on the semantics of the traditional C stack, and they didn't want to throw away their existing investments.
Where did this claim originate from? It gets tossed around all the time in greenthread discussions but it's completely false. When you create a real thread even though it has an 8MB stack or whatever it doesn't actually allocate 8MB. 8MB is not the allocation size, it's the growth limit. The stack grows dynamically allocating memory when necessary until it hits that limit. The C/C++/<insert any language here> ABI doesn't need to be compiled for this because it's just page faulting. Standard OS behavior for decades.
I think this might be caused by a lack of complementary programming concepts, such as actors.
Having worked with C# and the Orleans actor framework I found that I'm now using more complex async constructs, such as await Task.WhenAll() and async Linq statements that actually make code run more efficient.
Adding async and actors together to the Swift language seems like a very good idea to me.
It's easy to start a some task with a completion handler. Even error handling isn't that hard, since communication of a failure goes in the same direction as communicating success.
But cancellation goes into the other direction! So whenever you start an async operation, you need to somehow store a handle or something, so you can cancel it later on.
The actor model makes this even worse -- how do you cancel a command that you previously sent? How do you tell an actor that they don't need to perform an operation, if the operation is still on the queue, or that they should abort the operation if they are already working on it?
If the actor model doesn't have an answer to this problem, developers won't be able to use it as is, and they will have to build additional abstractions on top of it before they can use it.
It's inherently a half-measure: it's used to avoid wasting resources, but it only comes into play after you've already wasted resources. If you want to minimize waste, you're going to do better if you can minimize initiating operations that end up needing to be canceled.
Not that it isn't a useful refinement. It should be planned for. But I don't think it can be considered a critical feature out of the gate.
What if you are too late and, say, asynchronous write to a disk was already commited and the data is out of your program and on the drive?
And how do you cancel a sent email?
At some point there is no sense in sending a "cancel" message, or there is no way of cancelling an action. How would you proceed in such a situation?
Eg. imagine that a document is auto-saved in the background, but the network is busy. Then the user changes the document. Now the app wants to auto-save the new version, but the previous version hasn't been saved yet. Saving the previous version no longer makes any sense and would just waste time, so you want to cancel the previous save operation, and save the new version instead. (it doesn't matter if cancelling the previous save is successful or not -- all that matters is that you don't want to wait for the previous action)
Or imagine that an app opens and tries to show the last document that the user displayed. The document is big, so the app shows a loading indicator. But the user doesn't want to actually view the document, he wants to view a different document, so he closes the document and opens a different one. Now the app needs to cancel loading the first document, because otherwise it would take much longer to load the second document that the user actually wants to see.
Most of the posts that I see related to Swift are RFCs evaluating solutions to problems in other languages. I rarely get to see the actual solutions being integrated into the language. Is this just a result of HN readers caring more about language design than learning to leverage the Swift language?
It's a modern, fast, statically compiled, statically typed, language (that also has shims for interacting with legacy Obj-C code, but that's not a very important aspect) that unlike Go keeps up with modern PL features and expressibility, and unlike Rust has automatic RC-style memory management that you don't have to think about as much. There's no over-arching principle at play -- it's a pragmatic language.
>Why would I use Swift on Linux?
Because you like the actual language and its ecosystem (or not). It's not like you should be using any language just because of some "design philosophy" it's supposed to have.
>Most of the posts that I see related to Swift are RFCs evaluating solutions to problems in other languages. I rarely get to see the actual solutions being integrated into the language.
Well, there are several free books from Apple and tons of material to see what the language itself offers.
1. https://benchmarksgame.alioth.debian.org/u64q/compare.php?la... 2. https://medium.com/@rymcol/linux-ubuntu-benchmarks-for-serve...
1. Specifically for the first set of benchmarks linked to, they are really irrelevant because they don't represent typical workloads unless you're writing a MAAS (Mandlebrot As A Service). Node was designed for I/O efficiency, not fastest CPU-bound computations. That's exercising the JS engine (e.g. V8) more than anything node-specific. With node becoming more VM-neutral, it's entirely possible for other engines (e.g Chakracore or SpiderMonkey) to be better at other types of computation.
2. Especially in the second set of linked benchmarks, it seems the author of the article was hardly a node.js developer because they not only used an older node branch at the time they wrote the article, but they left out a lot of common optimizations (some of which were pointed out in the comments section). Even Express (which the author used) is known to not be very well optimized.
With that in mind, benchmarks are not the only thing you should be looking at IMHO. For example (for me personally), using a single language for frontend and backend is a big deal because there is less cognitive overhead when switching between the two (previously I often found myself writing JS syntax in PHP scripts and vice versa and trying to remember the APIs for different languages/platforms is difficult). There are many other benefits as well, just watch some of Mikeal Rogers' talks to get a better idea.
I also feel like with Swift people are so focused on pumping out their mobile app that best practices go out the window, or even that the knowledge is seen as such a valuable and proprietary skillset that the senior and experienced developers simply keep it to themselves and don't publish (compared to other OSS ecosystems)...
As evidenced by C#, you can't avoid leaking the type signature of async operations if you actually support generic programming- so while that's a nice ergonomics improvement, it only adds complexity to the actual concurrency model. Go enthusiasts out there will appreciate that go solves this by refusing to support PROGRAMMABLE generic abstractions at all (looking at you, channels and map).
Referencing the actor model and making it first class is interesting, but probably a mistake. Actors are hard to reason about because they're so flexible. Pony is a good recent attempt at combining static types with actors, bit they didn't put performance into the "non-goals" section of their language spec.
If you want task level concurrency and you want it to play nice with your type system, you have to start with Scala and work backward to the alternative implementation choices you're going to make because it checks all the boxes of all the "goals" and ALSO has a very mature actor model implementation that doesn't require promoting actors to keyword status in the language.
sudo is something different
I'm not saying it's bad, but I am saying that the flexibility limits the ability to reason about actor interactions (especially concurrent interactions), which makes it akin to the dynamic vs static type system debates. One is strictly more flexible than the other, and that necessarily makes it harder to reason about.
Did you have any specific improvements? The big requirements for distributed programming are mostly protobuf serialization and tcp/http; swift has both already.
Wrapping specialized interrupt handlers into some higher lebel API, such as AIO, is the right way.
Async/await is a mess. Concurrency cannot be generalized to cover all the possible cases. It must have specialization.
Engineers of old times who created the early classic OSes were bright people, contrary to current hipsters.
Once popularized, flawed abstractions and apis such as pthreads would stick (only idiots would accept shared stack and signals).
Look at how an OS implements concurrency and wrap it into higher level API. That's it.
How does the OS implement concurrency, which is a userspace problem?
Software interrupts alone are not enough, you still need some language support to it. I am not sure you really know what concurrency even means...
This is another gross misconception, like an attention to manage resources from the userspace, the way JVMs or Node are trying to do by poorly reimplementing OS subsystems.
Concurrency primitives must be implemented by an OS - it will eliminate all the userspace problems - no busy-waiting, no polling, no waking up for every descriptor, etc.
Ignoring an underlying OS instead of having thin wrappers, like Plan9 from userspace does, is exactly what is called ignorance (lack of awareness of a better way).